Tape recording systems employing multiple parallel longitudinal tracks recorded on e.g. one half inch tape are known. Each track typically extends for the entire useful length of the tape, which may be 1800 feet or longer. A head structure contains multiple read/write head elements. When user data is being recorded, a first set of tracks is recorded as the tape moves in a first or forward direction across the heads. When the end of the tape is reached, the head structure is repositioned, and a second set of tracks is recorded as the tape moves in a second or reverse direction across the heads. This back-and-forth recording process continues until the tape is completely filled up with user data, or until a host computer stops sending user data to the tape recording system.
A known coarse positioner mechanism is described in commonly assigned U.S. Pat. No. 5,105,322 to Steltzer, entitled: "Transverse Positioner for Read Write Head", the disclosure thereof being incorporated herein by reference. In this prior patent a moveable head carriage supported the tape head structure. The head carriage carried one or more read-write head pairs of the head structure. Eight write heads, and four read heads, were typically carried by the head structure, in a four-channel tape transport. The head carriage engaged a lead screw which was rotated by a stepper motor mounted to a base of the tape transport mechanism. The lead screw, and a parallel guidepost, enabled the head carriage to be movably positioned transversely relative to a direction of travel of the tape. Thus, by energizing the stepper motor, the head carriage was stepped across the track recording positions of the tape, as the tape streamed back and forth from end to end across the head structure during writing/reading operations.
The known coarse positioner operated in a quasi-open loop fashion in the sense that servo patterns were recorded at each end of the tape and were read and used for precisely positioning the head structure at nominal track centerline of the tracks being followed during an ensuing passage of the tape across the heads. However, while the known coarse positioner worked satisfactorily for tape track densities on the order of 256 tracks per inch, instantaneous lateral tape motion disturbances effectively limited the number of tracks that could be defined on the tape storage medium.
Some later improvement was obtained by the use of azimuth recording techniques, such that the head structure confronted the lineal tracks at a first azimuth angle during a pass from beginning of tape to end of tape, and at a second azimuth angle differing from the first during a reverse direction pass from end of tape to beginning of tape. The resultant data track recording patterns defined herringbone geometry and achieved a linear track density of e.g. 416 tracks per inch. An example of an azimuth recording system is given in commonly assigned U.S. Pat. No. 5,523,904 to the present inventor, entitled: "Linear Tape Write Servo Using Embedded Azimuth Blocks", the disclosure thereof being incorporated herein by reference. A first example of an azimuth head positioning mechanism is given in commonly assigned copending U.S. patent application Ser. No. 08/918,477 filed by Kasetty on Aug. 26, 1997, entitled: "Tape Head Positioning Device for Adjusting Head Tilt", and a second example of an azimuth head positioning mechanism is given in commonly assigned U.S. Pat. No. 5,680,278 to Sawtelle, Jr., entitled: "Apparatus for Combining Linear and Rotational Motion of an Azimuth Read-Write Head". The disclosures of the pending application of Kasetty and the patent of Sawtelle, Jr., are incorporated herein by reference.
While these prior approaches have worked to enable increases in track densities, a limitation with open loop or quasi closed loop positioning remained, due to lateral tape motion caused by a number of vibration excitation sources, including the supply reel and motor, take-up reel and motor, and guide rollers which guide the tape along a predetermined tape path across the head structure. These sources may separately or additively contribute to cause lateral tape motion. While prior efforts to reduce causes of lateral tape motion have been successful, as track densities increase (meaning that track widths are decreased) these prior efforts have reached practical limits, and have necessitated use of closed loop fine positioning servo mechanisms to provide relatively instantaneous adjustment of the tape head.
It has been proposed to combine a stepper motor as a coarse positioner with a linear voice coil motor acting as a fine track positioner to realize a head support structure capable of being positioned in closed loop during track following operations of the tape transport mechanism. One example of a dual actuator is provided by U.S. Pat. No. 5,280,402 to Anderson et al., entitled: "Combined Stepper Motor and Voice Coil Head Positioning Apparatus". In this prior approach, dual cantilever springs extended from a threaded nut structure to a head support structure. A lead screw rotated by a stepper motor engaged the threaded nut structure and thereby moved the head support structure coarsely across the tape during coarse head positioning operations. A linear voice coil motor was directly coupled to the head support structure and overcame a restorative bias spring force applied to the head by the dual cantilever springs. One drawback of this approach is that the springs were not stiff, but were flexible and susceptible to unwanted vibrations, requiring dampening structures or treatments.
A further prior art closed loop fine positioner for a tape head is disclosed in U.S. Pat. No. 5,379,170 to Schwarz, entitled: "Dynamically Adjustable Head Positioning Mechanism". This patent describes head carriage or stage which is coarsely positioned by a stepper motor lead screw relative to a tape transport base. The stage forms a lever which secures the tape head at one end and is pivotally mounted to a lead screw follower by a leaf spring which allows for longitudinal displacement of the head as well as pivotal displacement. An actuator attached to the stage rotates and thereby imparts limited rotational displacement of the head relative to the tape path to provide for fine position adjustments in real time. One drawback with this prior approach is lack of stiffness in that the leaf spring fails to isolate the head from vibrations which may be induced as the tape passes over the head. Another drawback with this prior approach is that the tape head does not remain perpendicular relative to the tape over the range of limited lateral displacement, thereby causing the tape to stretch and some of the reader-writer elements of the head to fail to register with previously-recorded tape tracks, particularly when the tape has been written by another tape transport not imparting identical rotational displacement to its head.
Thus, a hitherto unsolved need has remained for a coarse/fine dual actuator positioner which is much stiffer and less susceptible to unwanted vibrations than heretofore, and which avoids tape distortions otherwise resulting from rotating the tape head relative to the tape to provide fine position adjustments.